DB2 seems to do a SORT before every GROUP BY action. At least if I look
at the query plan (using Visual Explain in the Command Center) of
-- connected to the sample db
SELECT WORKDEPT, SUM(SALARY)

I called Stored Procedure written in SQL from Object REXX on Windows
like this
  CALL SQLEXEC 'CALL MigrateDECODE(:inString,:Converted)'
  CALL CHECKERR 'CALL MigrateDECODE'